Crude oil is produced from geological formations where it is in intimate contact with brine (salt water). As the oil and brine are produced, their movement through geological formations produces an emulsion of water-in-oil, wherein tiny droplets of water are suspended in a continuous phase of oil. Generally, the amount of water produced from the formation in the oil field ranges from 1-2% and may even be higher than 90%. Refineries operate with much lower water content in the crude oil, generally not exceeding 0.5%.
In oilfield industries, these water-in-oil emulsions are often referred to as primary emulsions. Though less common, oil-in-water emulsions, wherein tiny droplets of oil are suspended in a continuous phase of water, also occur and are often referred to as reverse emulsions. Another type of emulsion is a multiple, or complex, emulsion where tiny droplets are suspended in bigger droplets that are suspended in a continuous phase.
To render the crude oil more suitable for refining, the crude oil is demulsified by separating the primary or reverse emulsions into separate oil and water phases. Generally, the steps in demulsification are flocculation followed by coalescence and, finally, sedimentation. During the flocculation step, the suspended droplets aggregate to form larger droplets. During coalescence, the larger droplets come together to form a large drop. Sedimentation takes advantage of the fact that water is denser than oil. During sedimentation the water and oil phases become stratified into distinct layers as large drops of water fall to the bottom. There are several methods for demulsifying oil field emulsions, including thermal, mechanical, electrical, and chemical methods.
Chemical methods employ the use of chemicals that neutralize the effects of emulsion stabilizing agents and to accelerate the demulsification process by reducing the interfacial tension. These chemicals are often referred to as emulsion “breakers” because they break, or separate the emulsions into the separate oil and water phases. Chemicals used to break water-in-oil emulsions, or primary emulsions, are often referred to as primary emulsion breakers. Primary emulsion breakers are added to the continuous oil phase and are generally oil-soluble, though they may be water-soluble. Likewise, chemicals used to break oil-in-water emulsions, or reverse emulsions, are often referred to as reverse emulsion breakers. Reverse emulsion breakers are generally water-soluble, though they may be oil-soluble, and are added to the continuous water phase. Some of the water is removed from the crude oil by adding surfactant chemicals to demulsify the water and oil at the well or near the point of production. These surfactants are optimized to separate, or “break”, the oil and water at relatively low temperatures, common in the oil field.
While the water in the oil is a problem for refiners, it is the dissolved salts which cause the most problems since they can deposit and foul heat transfer surfaces. Calcium chloride and magnesium chloride decompose at operating temperatures of the refinery to produce HCl (hydrochloric acid) which corrodes the distillation towers. To remove the brine and the salts it contains, the crude oil is heated to around 120° C. and mixed with about 5% fresh water by passing the water and oil through a mixing valve and thence to a vessel, such as an oil refinery desalter, where it has a residence time of about 30 minutes to allow the emulsion to break and the oil and water to separate.
Without emulsion breakers, more time is required to separate the phases, limiting the amount of oil the refinery can process. In some cases, for example when a multiple emulsion is present, crude oil applications require both primary and reverse emulsion breakers. As primary emulsion breakers generally are oil soluble and reverse emulsion breakers are generally water soluble, the two types of emulsion breakers do not mix and are added to the crude oil or to the wash water separately.